1. Field of the Invention
The present invention relates to an AC/DC converter.
2. Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators etc., each receive externally applied commercial AC electric power for its operation. Also, electronic devices such as laptop computers, cellular phone terminals, and tablet PCs are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using such commercial AC electric power. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (converter) configured to perform AC/DC conversion of commercial AC voltage. Alternatively, such a converter is built into an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing an AC/DC converter 1r investigated by the present inventor. The AC/DC converter 1r mainly includes a fuse 2, an input capacitor Ci, a filter 3, a first rectifier circuit 4, a smoothing capacitor Cs, and a DC/DC converter 6r. 
The commercial AC voltage VAC is input to the filter 3 via the fuse 2 and the input capacitor Ci. The filter 3 removes noise included in the commercial AC voltage VAC. The first rectifier circuit 4 is configured as a diode bridge circuit which performs full-wave rectification of the commercial AC voltage VAC. The output voltage of the first rectifier circuit 4 is smoothed by the smoothing capacitor Cs, thereby generating a converted DC voltage VDC.
An insulated DC/DC converter 6r receives the DC voltage VDC via an input terminal P1, steps down the DC voltage VDC thus received, and supplies an output voltage VOUT stabilized to the target value to a load (not shown) connected to an output terminal P2.
The DC/DC converter 6r includes a control circuit 10r, an output circuit 7, and a feedback circuit 8. The output circuit 7 includes a transformer T1, a first diode D1, and a first output capacitor Co1. The feedback circuit 8 generates a feedback voltage VFB that corresponds to the output voltage VOUT, and supplies the feedback voltage VFB thus generated to a feedback terminal (FB terminal) of the control circuit 10r. 
The control circuit 10r includes a switching transistor M1, a driving circuit 20, a starter circuit 30, a clamp circuit 40, and an AC voltage detection circuit 50.
A drain (DRAIN) terminal of the control circuit 10r is connected to a primary winding W1. The source (SOURCE) terminal of the control circuit 10r is connected to an external detection resistor Rs. The switching transistor M1 is arranged between the DRAIN terminal and the SOURCE terminal. The driving circuit 20 switches on and off the switching transistor M1 according to the feedback voltage VFB.
The switching transistor M1 is switched on and off so as to step down the input voltage VDc, thereby generating the output voltage VOUT. The control circuit 10r adjusts the duty ratio of the switching operation of the switching transistor M1 so as to stabilize the output voltage VOUT to a target value, and to control the coil current Ip that flows through the primary winding W1 of the transformer T1.
The detection resistor Rs is arranged in series between the primary winding W1 of the transformer T1 and the switching transistor M1. A voltage drop (detection voltage) VCS is produced across the detection resistor Rs in proportion to the current Ip that flows through the primary winding W1 and the switching transistor M1. The control circuit 10r controls, based on the detection voltage VCS, the current Ip that flows through the primary winding W1.
A second diode D2 and a second output capacitor Co2 are connected to an auxiliary coil W3 of the transformer T1. A DC voltage VCC is produced across the second output capacitor Co2 according to the switching of the switching transistor M1. The DC voltage VCC thus generated is supplied to a power supply (VCC) terminal of the control circuit 10r. 
An internal circuit of the control circuit 10r such as the driving circuit 20 operates receiving the power supply voltage VCC generated at the VCC terminal. Before the startup of the DC/DC converter 6r, the power supply voltage VCC is zero. Thus, the control circuit 10r cannot start up using the power supply voltage VCC. In order to solve such a problem, the control circuit 10r includes a starter circuit 30 arranged between the DRAIN terminal and the VCC terminal. Before the start of the switching of the switching transistor M1 of the DC/DC converter 6r in the startup operation of the DC/DC converter 6r, the starter circuit 30 charges the second output capacitor Co2 connected to the VCC terminal using the input voltage VDC supplied via the primary winding W1. Thus, such an arrangement is capable of raising the power supply voltage VCC to a voltage level required to operate the control circuit 10r before the start of switching of the switching transistor M1.
In some cases, the voltage VCC at the VCC terminal becomes excessively high due to the charging of the starter circuit 30. The clamp circuit 40 clamps the voltage VCC of the VCC terminal such that it is equal to or lower than a predetermined upper limit voltage (e.g., 12 V). The clamp circuit 40 supplies the voltage VCC thus clamped to the internal circuits such as the driving circuit 20.
The AC/DC converter 1r is used in various kinds of environments. In some cases, the AC voltage VAC input to the AC/DC converter 1r is higher than the rated value or otherwise is lower than the rated value. If the DC/DC converter 6r is operated in such a situation in which the AC voltage VAC is outside the rated range, this leads to defective operation of the DC/DC converter 6r. 
In order to solve such a problem, the control circuit 10r is configured to have a function of monitoring the AC voltage VAC. A second rectifier circuit 9 performs full-wave rectification of the commercial AC voltage VAC. In some cases, the first rectified voltage VRECT generated by the second rectifier circuit 9 is higher than several hundreds of V. In this case, the first rectified voltage VRECT having such a high value cannot be input as it is to the control circuit 10r. In order to solve such a problem, the first rectified voltage VRECT is divided by means of resistors R11 and R12, and the voltage VIN thus divided is input to the input (VIN) terminal of the control circuit 10r. 
The AC voltage detection circuit 50 detects whether or not the amplitude AAC of the AC voltage VAC is within a predetermined range based on the second rectified voltage VIN at the VIN terminal. More specifically, when the amplitude AAC of the AC voltage VAC is lower than a threshold amplitude ABO, the AC voltage detection circuit 50 stops the operation of the DC/DC converter 6r. This function is also referred to as “a low AC input voltage protection (brownout protection) function”. Furthermore, when the amplitude of the AC voltage VAC becomes higher than the threshold amplitude ABO, the operation of the DC/DC converter 6r is resumed (brown-in function).
There are capacitance components such as a capacitor Ci between the positive electrode terminal and the negative electrode terminal across which the AC voltage VAC is to be input. After the AC/DC converter 1r is unplugged from an electrical outlet, a charge remains in the capacitance components. As a result, the DC voltage VDC does not immediately become zero. However, after the AC/DC converter 1r is unplugged from an electrical outlet, the AC/DC converter 1r employed in a power supply adapter or an electronic device is required to reduce the voltage across the positive electrode terminal and the negative electrode terminal to a predetermined voltage or less in a predetermined period of time. In order to meet this requirement, a discharging resistor Rdis is provided to the AC/DC converter 1r shown in FIG. 1 such that it is arranged between the positive electrode terminal and the negative electrode terminal.
The inventor has investigated the AC/DC converter 1r shown in FIG. 1, and has come to recognize the following problems.
In order to monitor the AC voltage VAC, the AC/DC converter 1r shown in FIG. 1 requires high-voltage resistors R11 and R12 each arranged as an external component of the control circuit 10r. Such an external resistor element leads to a problem of a high cost. Furthermore, such an arrangement requires an additional mounting area, which is also a problem.